1. Field of the Invention
This invention relates to a method for preparing polyalkyleneterephthalate molding compositions fortified with a filler, especially a glass fiber filler, which composition can contain other additives such as stabilizers, flameproofing agents, and the like. This invention relates to a process for preparing such molding compositions, the molding compositions themselves, a process for molding such molding compositions into a shaped article and the resultant shaped article. The resultant shaped articles have improved characteristics especially in respect of lifetime performance under certain tests.
2. Discussion of the Prior Art
Of the thermoplastic polyalkyleneterephthalates, poly-(1,4-butylene)-terephthalate, especially, has been gaining increasing importance because its fabricating characteristics are better than those of polyethyleneterephthalate. Poly-(1,4-butylene)-terephthalate, which will be referred to as PTMT hereinafter, is substantially easier to fabricate by a short-cycle injection molding method at low mold temperature to produce articles of stable dimensions.
It is also known that filler-fortified polyalkyleneterephthalates have a higher rigidity, thermal stability of shape and tensile and flexural strengths than unreinforced polyalkyleneterephthalates. If fibrous fillers of sufficient length are used, the tensile and flexural strengths are especially improved. On the other hand, the fillers reduce the elongation at failure and the impact toughness, resulting in greater brittleness.
In the case of PTMT, for example, whose viscosity number amounts to 150 cm.sup.3 /g, the impact toughness (measured in accordance with DIN 53,453) is reduced from "unbroken"to approximately 35 kJ/m.sup.2 and the elongation at failure (measured per DIN 53,455) is reduced from about 200% to about 2% by a content of 30% glass fibers by weight (initial length 6 mm) when the incorporation of the glass fibers is performed in a single-screw extruder with a temperature program of 285-295-275.degree. C. (from infeed point to die).
The tensile and flexural strengths of glass fiber-filled PTMT are high, but they cannot be fully utilized at high static and/or dynamic stresses, because, due to flow processes in the material, the life time is relatively short. Thus, a glass fiber-fortified PTMT produced by the incorporating process described above and having a viscosity number of 127 cm.sup.3 /g and a glass fiber content of 30% by weight, has a flexural strength per DIN 53,452 of 190 N/mm.sup.2 ; when a test specimen is stressed with a bending force of 175 N/mm.sup.2 the life time is 30 seconds.
This life time behavior limits the practical uses of filler-fortified polyalkyleneterephthalate compositions, especially polytetramethyleneterephthalate.
Important properties are improved in glass fiber fortified polyalkyleneterephthalates as the viscosity increases, i.e., with increasing molecular weight. Upon the incorporation of fortifying fillers into polyalkyleneterephthalate, especially PTMT, even in the case of extruder screws having a gentle kneading characteristic, a thermal degradation (reduction of the viscosity number) of the polymer is encountered, which is greater as the viscosity of the polyalkyleneterephthalate), especially PTMT, increases. Due to the reduction of the viscosity number upon the incorporation of the glass fibers, an embrittlement takes place, resulting in considerable limitations in the application of the material.
In German Auslegeschrift No. 20 42 447 it is proposed to prepare filler-fortified polybutyleneterephthalates or polypropyleneterephthalates of a limited viscosity number from 0.2 to 1.2 dl/g (corresponding viscosity number range from about 20 to 160 cm.sup.3 /g) either by mixing the fortifying fillers with the polyalkyleneterephthalates after the polycondensation has been completed, e.g., by dry mixing or by mixing with the fully condensed polyalkyleneterephthalate in the melt phase, or by adding the fortifying fillers to the monomer before the beginning of the polycondensation reaction.
If the fortifying fillers are added to the polyalkyleneterephthalate) by dry mixing only, the fortifying fillers will not be uniformly distributed in the finished article. If the fortifying fillers are incorporated into the melt, the disadvantages described above (degradation of the polymer) are encountered.
The incorporation of glass fibers by mixing the fortifying filler into a mixture of the PTMT raw materials also leads to difficulties during the esterification and subsequent melt condensation. In private experiments it was found that the trouble-free incorporation of glass fibers, e.g., 30 percent glass fibers by weight, in the reaction vessel was impossible. Difficulties were encountered even during the transesterification. As the condensation progressed, it was observed that the glass fibers were not incorporated by the increasingly viscous melt and migrated to the surface. The degassing of butanediol-1,4 was considerably impeded by the agglomerating glass fibers. Stirring and mixing them in was virtually impossible or was possible only with great difficulty using special apparatus.
It is an object of this invention to provide a filler-fortified molding composition from, especially, poly-(1,4-butylene)-terephthalate which can be fabricated to tougher products of improved life time in comparison to the formerly known, comparable molding compositions.